feat(installer): pull in preflight check implementations as prerequisite for SystemCheckPage

Git/GitIdentity/Port/WriteAccess and Claude CLI/Version/Auth/PermissionModeAuto
checks plus the ExecutableResolver they depend on were built in two sibling
task branches that hadn't landed on main yet. Vendored the finished files in
from those branches (same content, verified building + tests green) so the
SystemCheckPage task has something to consume.

src/ClaudeDo.Installer/Core/Interfaces/IProcessRunner.cs

@@ -0,0 +1,6 @@
+namespace ClaudeDo.Installer.Core.Interfaces;
+
+public interface IProcessRunner
+{
+    Task<(int ExitCode, string Output)> RunAsync(string fileName, string arguments, string? workingDirectory, CancellationToken ct);
+}

src/ClaudeDo.Installer/Core/ProcessRunnerAdapter.cs

@@ -0,0 +1,9 @@
+using ClaudeDo.Installer.Core.Interfaces;
+
+namespace ClaudeDo.Installer.Core;
+
+public sealed class ProcessRunnerAdapter : IProcessRunner
+{
+    public Task<(int ExitCode, string Output)> RunAsync(string fileName, string arguments, string? workingDirectory, CancellationToken ct) =>
+        ProcessRunner.RunAsync(fileName, arguments, workingDirectory, progress: null, ct);
+}